Why the air slipping into a Charleston house deserves as much attention as the insulation, where the leaks usually hide, how a blower door puts a number on them, and the safety checks that come with tightening a house.
Insulation and air sealing do different jobs. Fiberglass and cellulose slow the heat that soaks through a ceiling, but air passes straight through them, carrying heat and water vapor along. In Charleston the water is the costly part: the state climatology office puts average August relative humidity at 79%, and every cubic foot of muggy air that leaks in has to be cooled and dried by the air conditioner.
So the work here runs in a fixed order: find the leaks, seal them, then insulate. It starts with a free assessment, a measured R-value and photographs that stay with you. On jobs that need a number, the blower door runs once before any sealing and once after it.
Two loads reach your air conditioner: heat and moisture. Air leaking in through the attic floor, the crawl space or the walls brings both, and a Lowcountry summer seldom supplies dry air. Charleston International Airport averages 64.3 days a year at 90°F or hotter in NOAA's 1991-2020 normals, and its cooling degree days (2,434) outnumber its heating degree days (1,844).
Winter leaks cost money too, because most houses here heat with electricity: about 76% of occupied homes in Charleston County, 77% in Berkeley and 75% in Dorchester, per the Census Bureau. The airport sees 22.4 nights a year at or below freezing, and on those nights a leaky house draws cold air in low and lets warm air escape up high.
For a sense of scale, EPA modeled a typical house from the 1970s and 1980s, assumed the sealing would cut its total air leakage by a quarter alongside added insulation, and put the savings in southern climate zone 3 at about 14% of heating and cooling costs. A real house lands above or below that depending on where its leaks are, which is why they get found before they get priced.
ENERGY STAR's attic sealing guide says the attic is usually where the biggest savings sit, and its list of common leak points works as a map for the rest of the house too:
The leaks you can feel at a door are rarely the whole story. The hidden ones at the top and bottom of the house are where a blower door and a flashlight earn their keep.
A blower door is a calibrated fan fitted into an exterior doorway with the real door propped open. PNNL's Building America guide to enclosure leakage testing describes the usual method: the fan pulls air out until the house sits at negative 50 pascals compared with outdoors, and the airflow it takes to hold that pressure, in cubic feet per minute, is the result (CFM50).
Dividing by the size of the house gives a figure you can compare: air changes per hour at 50 pascals, or ACH50, which is CFM50 times 60 divided by the house's volume in cubic feet. A 2,000-square-foot house with 8-foot ceilings holds 16,000 cubic feet, so a reading of 2,400 CFM50 works out to 9 ACH50.
The same guide lists the preparation: windows latched, gas water heaters turned down to pilot, fireplace ash removed or covered. It also warns that pulling a house to negative pressure can draw contaminants in from walls, attics and crawl spaces, so a house with suspect insulation, vermiculite included, may need remediation first or a test run with the fan pushing air in instead. Where the job calls for it, the reading is taken before the sealing and again after, so the improvement is measured rather than assumed.
One product does not fit every hole. ENERGY STAR's attic sealing project sorts the materials by gap size and location:
| Gap | Material |
|---|---|
| Cracks up to about 1/4 inch | Silicone or acrylic latex caulk |
| Gaps from 1/4 inch to about 3 inches | Expanding spray foam |
| Around metal flues and masonry chimneys | High-temperature caulk and aluminum flashing, never spray foam |
| Dropped soffits, chases and other large openings | Rigid blocking such as reflective foil insulation, sealed at the edges |
| Attic hatch and exterior doors | Weatherstripping, plus insulation on the hatch itself |
Flues are the one place a mistake becomes a fire risk. ENERGY STAR points out how hot a furnace flue gets, and that the usual code gap is an inch around a metal flue and two inches around a masonry chimney; the sheet-metal collar is there to hold that gap open while the hole around it gets closed. For rim joists and other spots where sealing and insulating can happen in one pass, closed-cell foam is often the tool; see spray foam.
A tighter house changes how air moves through it. ENERGY STAR warns that sealing can, in some cases, cause naturally venting combustion appliances to backdraft their exhaust into the living space, and it advises testing those appliances for proper drafting before and after the work. Across the three counties about one occupied home in five heats with utility gas, and a few more with bottled gas, per the Census Bureau.
The same guidance says some houses need added mechanical ventilation after sealing to keep the indoor air safe. Exhaust fans that actually reach the outdoors are part of that, and PNNL's guide for existing attics requires bath fans that empty into the attic to be ducted outside.
In an all-electric house the backdraft question mostly falls away, but the fresh-air question does not. If your house burns gas, ask how drafting will be checked before and after, and get the answer in the written scope.
Ducts that run through an attic or crawl space push conditioned air out of their joints and, on the return side, pull attic or crawl space air in. For new construction the 2009 code requires a duct tightness test unless the air handler and every duct sit inside the conditioned space. The City of Charleston's code compliance form asks the contractor to name who tested the ducts and who verified the envelope's air sealing.
Duct sealing is heating and cooling work, and the utilities pay for it separately from insulation:
We seal the house itself; the ducts go to whoever services the system, and the assessment photographs their condition so you know what to ask for.
The 2009 IECC, South Carolina's statewide energy standard, requires a new house to be sealed against air leakage and lets the builder show it either by testing or by visual inspection; recessed lights have to be sealed as well. On an existing house it reaches only a renovation, which the statute defines as repairs or alterations adding up to over half of what the building is worth inside one year.
In a mapped flood zone, sealing work can count toward a bigger number. Sullivan's Island totals three years of improvement costs for its 50% flood rule, and its worksheet lists insulation and weather-stripping by name. The City of Charleston totals one year in zones AE and VE and requires a permit there for all work on an existing home, interior jobs included; unincorporated Charleston County totals five years. A large sealing and insulation job lands in that tally with everything else, so it is worth knowing where the house stands before the work is scheduled. Permitting is on us.
Air sealing is priced by the work, not the square foot: how many penetrations there are, how much insulation has to be pulled back to reach them, how cramped the attic is, and whether chases and soffits need rigid blocking. The written scope lists each area separately, so you can see what the sealing line covers.
For Dominion electric customers who heat with electricity, Home Energy Check-up Plus is the big one: through March 31, 2027 its limited-time offer covers 90% of air sealing, attic insulation and duct sealing combined, up to $2,880, after the free Home Energy Check-up and only through the program's approved contractor. Berkeley Electric's HomeAdvantage loan finances air-infiltration work at 5% interest on the power bill. Palmetto Community Action Partnership's weatherization is free for households that meet the income limits. Each of those is checked before you get a quote from us.
A house can end up under-ventilated. ENERGY STAR notes that tightening a house may call for added mechanical ventilation to keep the indoor air safe, and that naturally venting gas appliances should be tested for drafting before and after. The answer is controlled fresh air from fans that reach the outdoors, not random leaks.
South Carolina's code sets its airtightness rules for new construction, not for existing houses. For a house you already own, the useful figures are its own readings before and after the sealing, which show how much the work changed. Bigger drops usually come from the attic floor and the floor over the crawl space.
Yes, a good share of it. ENERGY STAR publishes do-it-yourself projects for sealing the attic and sealing behind window and door trim. Its cautions are worth following: no spray foam around flues, walk only on joists or truss chords, and leave anything that looks like vermiculite undisturbed.
It helps, but ENERGY STAR's attic sealing guide points to the attic as where the largest savings usually are. Sealing the attic floor and the floor over a crawl space comes first for that reason, with doors and windows after.
It cuts how much damp outdoor air gets in, which leaves the air conditioner less moisture to remove. It will not dry out a house with a roof leak or a wet crawl space, and a tighter house still needs bath fans that vent outdoors.
Air sealing across greater Charleston, from the peninsula and the sea islands out to Summerville, Goose Creek and Moncks Corner. Each city has its own page with the permit rule and code edition that apply there:
Sources: SC State Climatology Office: August relative humidity, Charleston (79%) · NOAA 1991-2020 normals, Charleston International Airport · Census ACS 2020-2024: house heating fuel by county (B25040) · ENERGY STAR savings methodology (climate zone 3 estimate, 25% leakage reduction assumed) · ENERGY STAR attic air sealing project: leak list, materials by gap size, flue clearances, backdrafting, ventilation · ENERGY STAR: seal behind window and door trim · PNNL Building America Solution Center: enclosure leakage (blower door) test, CFM50 and ACH50, preparation, contamination warning · PNNL Building America Solution Center: blown insulation in existing vented attics (bath fans vented outside) · 2009 IECC for South Carolina (Insulation Institute): air sealing by test or visual inspection, duct testing, air handler and cavity sealing · South Carolina Energy Standard Act (renovation defined) · City of Charleston IECC contractor compliance form (duct test and envelope verification parties) · City of Charleston substantial improvement rules in flood zones AE and VE · Town of Sullivan's Island 50% improvement worksheet · Charleston County floodplain management (five-year permit total) · Dominion Energy SC ductwork rebate · Santee Cooper duct replacement rebate · Dominion Energy SC Home Energy Check-up and Check-up Plus · Berkeley Electric Cooperative HomeAdvantage loan · Palmetto Community Action Partnership weatherization. Codes, programs and prices change; confirm the current rule with your city before you rely on it.
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